Foundations of Geospatial Analysis

Adam Dennett

About Me

  • Professor of Urban Analytics & Current Head of Department @ Bartlett Centre for Advanced Spatial Analysis (CASA), UCL

  • Geographer by background - ex-Secondary School Teacher - back in HE for 15+ years

  • Taught GIS / Spatial Data Science at postgrad level for last 10 years

About this session

  • Whistle-stop tour of some of the key concepts relating to spatial data

  • Some examples of how carrying out analyses of spatial data can require additional attention - “spatial is special”

    • An illustrative example of analysing some spatial data for London

Key Geospatial Concepts

  • Where? (absolute)
  • Where? (relative)
  • How near or distant?
  • What scale?
  • What shape?

Where? (absolute)

  • Everything happens somewhere

    • We’re here: Wallspace, 22 Duke’s Road, Camden, London, England, *Europe, Northern Hemisphere, Earth

Where? (absolute)

  • How do we know exactly where?

XKCD - No, The Other One

https://xkcd.com/2480/

Where? Coordinate Reference Systems

  • More reliable than names (that are rarely unique or reference fuzzy locations), are coordinates

  • The earth is roughly spherical and points anywhere on its surface can be described using the World Geodetic System (WGS) - a geographic (spherical) coordinate system

  • Points can be referenced according to their position on a grid of latitudes (degrees north or south of the equator) and longitudes (degrees east or west of the Prime - Greenwich - meridian)

  • The last major revision of the World Geodetic System was in 1984 and WGS84 is still used today as the standard system for references places on the globe.

https://www.earthdatascience.org/courses/use-data-open-source-python/intro-vector-data-python/spatial-data-vector-shapefiles/geographic-vs-projected-coordinate-reference-systems-python/

Where? Coordinate Reference Systems

  • Projected Coordinate Reference Systems convert the 3D globe to a 2D plane and can do so in a huge variety of different ways

  • Most national mapping agencies have their own projected coordinate systems - in Britain the Ordnance Survey maintain the British National Grid which locates places according to 6-digit Easting and Northing coordinates

  • Every coordinate system can be referenced by its EPSG code, e.g. WGS84 = 4326 or British National Grid = 27700 with mathematical transformations to convert between them

Where? Coordinate Reference Systems

  • Once we have a coordinate reference system we can locate objects accurately in space

  • Most objects that spatial data scientists are concerned with (apart from gridded representations, which we will ignore for now!) can be simplified to either a point, a line or a polygon in that space

  • Polygons and lines are just multiple point coordinates joined together!

Where? Relative - Tobler’s First Law of Geography

“Everything is related to everything else, but near things are more related than distant things.”

  • This observation underpins much of what spatial data scientists do

  • Being able to locate something in space allows us to:

    • explain why something may be occurring where it is

    • make better predictions about nearby or further away things

Where? Relative - Defining ‘near’ and ‘distant’

  • Near and distant can mean different things in different contexts

    • the farthest one would travel to buy a pint of milk is somewhat different to farthest one might be willing to commute for a job
  • In spatial data science one way of separating near from distant can simply be to define their topological relationship

  • The Dimensionally Extended 9-Intersection Model (DE-9IM) is the standard topological model used in GIS to define spatial relationships between objects

Where? Relative - Exploring Near and Distant

  • If we take London and all of its Wards we can begin to explore how we can define those that are near and those that are distant from each other

Where? Relative - Exploring Near and Distant

  • If we measure the distance from the centre (centroid) of one ward to another, then we might decide that the 1st, 2nd 3rd, kth. closest wards are near, the others are far.

  • We can then decide to include the “k” nearest neighbours or exclude the rest

Where? Relative - Exploring Near and Distant

Where? Relative - Exploring Near and Distant